This invention pertains generally to electrolytes and particularly to hybrid thin film polymer electrolytes for lithium rechargeable batteries.
Lithium-based rechargeable batteries offer significant advantages over other rechargeable electrochemical systems in terms of improved performance characteristics such as increased power and energy density and cycle life, particularly for consumer applications such as computers, power tools, etc. For this reason, a significant amount of research is aimed at improving the characteristics of rechargeable lithium batteries. Of particular interest, is the development of improved electrolytes.
Aprotic liquid electrolytes used in lithium rechargeable batteries possess the high conductivities necessary for high energy and power density applications, however, they suffer from several significant disadvantages. Batteries having liquid electrolytes are prone to leak and consequently, present a safety problem; they suffer from anode passivation which reduces output and life; and because they cannot be produced by continuous processes, manufacturing is inefficient. Solid electrolytes overcome most of these disadvantages except that they have low conductivities at normal operating temperatures (.ltoreq.10.sup.-5 ohm.sup.-1 cm.sup.-1), limiting the current that can be withdrawn from the battery.
Hybrid thin film electrolytes, which are basically liquid electrolytes in which the liquid phase has been immobilized by incorporation into a polymer, combine the best features of both liquid and solid electrolytes; retaining the high conductivity of liquid electrolytes and the electrolyte immobility of solid electrolytes while overcoming the disadvantages of both liquid and solid electrolytes. Because of their attractive features these electrolytes have been studied extensively and demonstration batteries incorporating hybrid thin film electrolytes have been built. However, the ambient temperature conductivities have generally been too low to be useful for most battery applications. Prior attempts to fabricate hybrid thin film electrolytes and the use of these electrolytes in batteries has been summarized in K. M. Abraham, Ambient Temperature Polymer Electrolyte Batteries, Fourth Annual International Rechargeable Battery Seminar, Deerfield Beach, Fla., Mar. 3, 1992 and B. Barnett and D. Fauteux, New Directions in Polymer Electrolyte Battery Technology, ibid.
In these hybrid thin film electrolytes, the immobilizing polymer phase comprises two basic forms; chemically crosslinked polymers and polymers that can form a physical network, i.e., capable of forming a gel based on physical interactions among their linear chains. The electrolyte, contained within the polymer phase, generally comprises a lithium salt and an aprotic solvent or mixture of solvents capable of dissolving the lithium salt. Because of their good mechanical properties, these hybrid electrolytes can be cast as thin films that allow ions to move through them when a potential is applied between the electrodes on which the film is deposited.
It is well known to those skilled in the electrochemical art that the ionic conductivity of an electrolyte is strongly dependent upon the concentration of the solute. Consequently, it is advantageous to raise the concentration of the solute in the electrolyte until an optimum concentration is reached, typically that concentration of solute at which the solution exhibits a maximum conductivity. Propylene carbonate has been used as a solvent for lithium salts because of its high dielectric constant. As shown by Abraham et al. (J. Electrochem. Soc., 137, 1657-1658, May 1990), the conductivities of hybrid thin film electrolytes having a propylene carbonate solvent can be as high as 10.sup.-3 ohm.sup.-1 cm.sup.-1 However, propylene carbonate has a high vapor pressure relative to other high dielectric constant organic solvents, such as succinonitrile, which makes propylene carbonate less desirable for use in battery applications. Studies of the conductivity of hybrid electrolytes have generally employed LiClO.sub.4 as a solute, e.g., Watanabe et al. (J. Polymer Science, 21, 939-948, 1983). However, because it is a strong oxidizing agent, LiClO.sub.4 is unacceptable as a solute for battery applications for safety reasons. What is needed is a hybrid thin film electrolyte, useful for lithium rechargeable batteries, having a conductivity .apprxeq.10.sup.-3 ohm.sup.-1 cm.sup.-1, and in which the liquid phase contained therein comprises a good solvent for lithium salts, has a low vapor pressure at operating temperatures that would be experienced by rechargeable lithium batteries and a lithium containing solute which does not present safety hazards.
Responsive to these needs, the instant invention discloses novel hybrid electrolyte thin films which are compositionally stable, environmentally safe, can be efficiently produced in large quantities and which, because of their high conductivities .apprxeq.10.sup.-3 ohm.sup.-1 cm.sup.-1 are useful as electrolytes for rechargeable lithium batteries.